In many Earth engineering applications, wells are drilled into subterranean porous formations. It is desirable to create a fracture connecting two neighboring wells. In general, the fracture follows the plane perpendicular to the least resistance, i.e., perpendicular to the original in-situ minimum stress, Smin. Thus, normally, the two wells need to be drilled so that the line connecting them is aligned perpendicular to Smin. Otherwise, if the two wells are drilled substantially deviated from the preferred direction, a fracture may not be formed to connect the two wells. In Canada and many parts of the world, petrochemicals are found in heavy, viscous forms such as bitumen, which are difficult to extract. The bitumen-saturated oilsands reservoirs of Canada, Venezuela and California are just some examples of such subterranean formations. In these formations, it is not possible to simply drill wells and pump out the oil. Instead, the reservoirs are heated or otherwise stimulated to reduce viscosity and promote extraction.
The two most common and commercially-proven methods of stimulating oilsands reservoirs are (a) cyclic steam stimulation (CSS) and (b) steam assisted gravity drainage (SAGD). In both cases, steam is injected into the reservoir, to heat up the bitumen. Some variations of these processes may involve injecting solvent to aid the viscosity reduction or use electrical heating to replace the role of steam. In general, the initial injectivity into the reservoir, i.e., how much volume of the stimulant can be injected per unit of time, is relatively small. Fracturing of the reservoir is desired to provide channels for the stimulant travel and to access the reservoir. The fracture not only increases the injectivity, but also increases the contact area of the stimulant within the reservoir. For example, in CSS, the injection pressure goes above the reservoir's fracture pressure with the goal to form the fracture. It is desirable to be able to control the orientation, depth and length of fractures in the reservoir, in order to more effectively place stimulant in the targeted location, extent and/or time, all of which can help maximize petroleum extraction.
In the SAGD process, before the production can start, communication between the SAGD well pair must be established so that the bitumen can flow down to the production well. Conventionally, steam is circulated through the said two wells independently until the inter-well area is heated and the bitumen viscosity is reduced significantly so that it can flow to the production well and communication is established. This process normally takes up to 6 months to complete. Such a non-productive period wastes steam and manpower, ties up the capital used to build the infrastructure. If the SAGD wells can be hydraulically fractured, forming a high-mobility conduit connecting the two SAGD wells, the inter-well communication can occur much earlier and stronger.
The art of hydraulic fracturing as a stimulation method for hydrocarbon resource recovery has been practiced for a long time. In general, this method injects liquid at a high pressure into a well drilled through the target formation to be stimulated. The high pressure initiates a fracture from the injection well and propagates a sufficient distance into the formation. Then, the fracture is filled with proppants that are injected from the surface after the fracture is formed. The similar method is applied in vertical and horizontal wells and wells of any inclinations. However, the existing art of hydraulic fracturing is subject to limitations.
In hydraulic fracturing, there has historically been no proactive control of the orientation of the fracture formed. The fracture typically follows the plane perpendicular to the least resistance, i.e., perpendicular to the original in-situ minimum stress, Smin. In many situations, SAGD wells may not be drilled in this optimal direction. For example, the azimuth of the SAGD wells being drilled might be dictated by the deposit channel of the oilsands resource. The well pair then tends to follow the channel direction which may or may not coincide with the Smin direction. If a horizontal well is drilled in the direction of the minimum stress Smin or substantially inclined towards it, the fracture being formed via the conventional hydraulic fracturing may be discrete in the vertical cross-section perpendicular or substantially perpendicular to the horizontal well. Such fractures may not be ideal for the petroleum production. For example, discrete fractures perpendicular to the SAGD wells do not contribute to uniform communication between the well pair.
There has been some work done in controlling the orientation of fractures including selective placement of hydraulically-driven fractures in the plane perpendicular to the original in-situ maximum stress, Smax. These practices in the past, however, typically require a sacrificial well which was fractured first along the direction perpendicular to Smin, i.e., the original in-situ stress condition dictates the fracture formed on this sacrificial well. For example, U.S. Pat. No. 3,613,785 by Closmann (1971) teaches creating a horizontal fracture from a first well by vertically fracturing the formation from a second well and then injecting hot fluid to heat the formation. Heating via the vertical fracture alters the original in-situ stress so that the vertical stresses become smaller than horizontal stresses, thus favouring a horizontal fracture being formed. This method requires a first sacrificial vertical fracture be formed and uses costly steam to heat the formation.
U.S. Pat. No. 3,709,295 by Braunlich and Bishop (1971) controlled the direction of hydraulic fractures by employing at least three wells and a natural fracture system. This method is only feasible in formations already having existing fractures.
U.S. Pat. No. 4,005,750 by Shuck (1975) teaches creating an oriented fracture in the direction of the minimum in-situ stress from a first well by first hydraulically fracturing another well to condition the formation. Again, additional wells and sacrificial fractures are required before the targeted fracture can be formed.
Canadian patent CA 1,323,561 by Kry (1985) teaches creating a horizontal fracture from a center well after cyclically steam-stimulating at least one peripheral well. At the peripheral well a vertical fracture is created. CSS operations coupled with fracturing at the peripheral well conditions the stress field so that a horizontal fracture can be formed. To create the horizontal fracture, a high-viscosity fluid is proposed to inject into the center well to limit the fluid from leaking into the formation.
Canadian patent CA 1,235,652 by Harding et al. (1988) first vertically-fractures the formation from peripheral wells to alter or condition the in-situ stress regime in the center region of the peripheral wells. The formation is then fractured through a central well to create and extend a horizontal fracture.
All of the above documents require either the existence of a natural fracture in the formation already or the formation of sacrificial fractures before a targeted fracture can be induced. This pre-condition adds cost to well drilling and completion.
The idea of forming a target fracture without initiating sacrificial fractures has been proposed in two presentation papers by Lessi, J., et al. . (“Underground Coal Gasification at Great Depth”; Technical Committee of Groupe d'Etude de la Gazefication Souterraine du Charbon and “Stress Changes Induced by Fluid Injection in a Porous Layer Around a Wellbore”; 24th US Symposium on Rock Mechanics June 1983). These papers propose drilling two wells and forming a fracture connecting them even though their connection line may be not oriented perpendicular to Smin. According to the authors, this process relies on pressure diffusion and thus-associated poroelastic stress to create a fracture between the two wells. The two papers did not address interaction between the wells.
It is therefore of great interest to find a new method to over-come the original in-situ stress condition for selective placement of a fracture without drilling a sacrificial well or dictating presence of natural fractures.